The present invention relates to a light spot position control system and method by a sampling servo in which a light spot position control to perform a focusing control and/or a tracking control of a light spot which is focused onto a recording medium is executed on the basis of servo signals which are derived as sampled signals from servo areas which are intermittently provided in the recording medium.
In an optical memory device for writing data into an arbitrary recording area (data area) on a recording medium such as an optical disc or the like and for reading out data from an arbitrary recording area, it is necessary to perform a light spot position control to accurately control the positional relation between the recording surface and a light spot which is focused onto the recording surface. As such a kind of light spot position control, an auto-focusing control for allowing a light spot to track the vertical motion of a recording medium and a tracking control for allowing a light spot to track the movement of a track on a recording medium have been known. Recently, there has been proposed a sampling servo system in which a servo area and a data area are alternately formed along a track on a recording medium, a position of a light spot is controlled using servo signals which are derived as sampled signals from the servo areas, and data is recorded and reproduced data into/from the data areas. As an example of a light spot position control system of the sampling servo type, there have been known U.S. Pat. No. 4,564,929 and "Data File Optical Disc of the Sector Servo System", the Abstract for the 45th meeting of The Japan Society of Applied Physics, 13P-E-8 and 13P-E-9.
It is a feature of the sampling servo system that in a servo system of a focusing control and a tracking control, servo signals (error signals) are not continuously detected but are obtained as sampled signals from the servo areas which are intermittently formed, and these servo signals are held for an interval corresponding to the data areas. Therefore, there is an advantage such that the servo signals are not influenced by the data areas. For example, in the recording mode to record data pits into a data area, in general, a recording pulse light of an intensity which is about ten times as powerful as that in the reading mode enters a photo detector. Thus, in the case of a continuous control, there is a fear such that the servo system for focusing and tracking is disturbed by the recording pulse light. However, in the sampling servo system, since the signals other than the signals detected at sample points are not used, the servo system is not influenced by the recording pulse light. On the other hand, in general, in an optical memory device such as an optical disc or the like into which data pits are recorded by a thermal energy of a light spot, the optical characteristics such as a reflection factor and the like in a data area after recording change as compared with those before recording. Therefore, the characteristics of the control system in the case of a continuous control are changed by those influences. However, in the sampling servo system, it is not influenced by a characteristic change of the data area and there is an advantage such that the servo system can be stabilized.
However, a conventional sampling servo technique is constructed on the assumption that the servo signals were always correctly detected and does not take a sufficient countermeasure for a detection error (sampling error) of servo signals due to scratch, dirt, or defect of the recording surface, electrical noises of the servo system, or the like. Therefore, there is a problem such that if the servo signal has a detection error, this error is not corrected but remains in at least the next servo signal to be sampled, so that the control characteristics of the servo system are disturbed and a control system having a predetermined performance cannot be constituted. For example, in the tracking servo system, even when the erroneous sample value was held for only one sample period, this state is equivalent to that a large acceleration was applied to an actuator for tracking, so that a light spot is deviated from the center of the track for a long time.